I've seen some truly yellow tennis balls so it probably depends on the brand, but I assume you're talking the yellow-green ones, it comes down partly to linguistics. Just like 'blue' the range when we say 'green' is bigger than we might expect. A lot of things we thing of as green are really yellow green and I bet if people were asked to pick a "middle" green it would be closer to yellow than cyan. Things like 'grass green' are much more yellow than you might imagine and when editing photos I've taken to looking in the yellow channels first to adjust green grass and trees.

They were never really white. I remember them being a very light fawn colour - sort of light grey with a hint of brown. I suppose the new ones were designed to be very visible against various tennis court colours - including grass, so yellow would seem more sensible than green. I've seen a lot of bright pink ones used at local courts - but I guess they're not approved for use in major tournaments.